Spark plug



. end threaded for insertion in the engine.

Patented Apr. 28, 1942 SPARK PLUG Taine G. McDougal, Flint, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application July 24, 1940, Serial No. 347,091 4 Claims. (01. 12s 1s9) This invention has to do with an improvement in spark plugs such as are used in internal com bustion engines.

Spark plugs occasionally fail because the electrical stress at the-insulator surface outside of the combustion chamber becomes so great that the contiguous air is ionized and the electrical discharge takes the form of a brush or corona discharge between the spark plug terminal and the insulator shell instead of a spark between the electrodes, This difficulty is increased where the insulator is made of material having a high dielectric constant. Some of the improved insulator materials, such as sintered oxides and combinations of sintered oxides and silicates, while far superior to conventional porcelain insulators in every other respect, possess the disadvantage of having a relatively high'dielectric constant.

It occurred to me that this difliculty could be overcome by employing in series with the insulator material, another material having -a low dielectric constant thereby greatly reducing the electrical stress at the surface of the insulator and hence avoiding any possibility of short-circuiting the spark through brush or corona discharge at the insulator surface. This may be simply accomplished by inserting the low dielectric constant material in the bore of the insulator surrounding the electrode. The material may consist of glass fused in place in the insulator bore or, if desired, the material may take the form of powder rammed in place.

The invention will be disclosed in detail in the course of the following description.

Figure 1 is a longitudinal vertical section through an insulator embodying my invention.

Figure 2 is a fragmentary view showing a slight modification.

Ill indicates an insulator mounted in any desired manner in the shell l2 having its lowe: indicates the usual side electrode and I6 indicates the composite center electrode here shown as consisting of a lower portion [8 made of heat and corrosion resistant metal or'alloy, and an upper portion 20, preferably made oi'good'heatconducting metal such as copper, connected by an intermediate portion 22 consisting of electrically conducting glass thermally bonded to the electrode sections and to the insulator. This.

type of composite electrode is described and claimed in Patent No. 2,106,578v "issued to schwartzwalder and Kirk on January 25, 1938 and in application Serial No. 343,808, filed by Schwartzwalder andRulka on July 3, 1940. The

' glass may be fused in place at the same time that the conducting glass 22 is fused in place. If preferred it may be in the form of powder tamped in position. Instead of glass, other suitable materials of low dielectric constant may be employed such as porcelain or various synthetic or natural insulating materials. Introduction of material of low dielectric constant between the central electrode and the shell substantially reduces the electrical stress at the surface of the insulator and so prevents brush or corona discharge which interferes with sparking at the electrodes.

In the modification shown in FigureZ, the electrcde I6 consists. of two sections integrally formed or welded together or simply held in close contact. The material 26 of low dielectric constant performs the same function as in Figure 1.

Obviously, if preferred, the material of low dielectric constant could be provided around the outside of the insulator instead of within its bore.

I claim:

1. A spark plug comprising spaced electrodes, an insulator between the said electrodes, and material having lower dielectric strength than the insulator interposed between one of said electrodes and the insulator so as to reduce the electrical stress in the insulator and prevent corona discharge or flashover.

2. A spark plug comprising spaced electrodes, an insulator between said electrodes having a portion the surface of which is normally exposed to the air, a material having lower dielectric strength than the insulator interposed between said portion ofithe insulator and one of said electrodes so'a's to reduce the electrical stress at the exposed surface of the insulator and prevent r corona discharge or flashover.

3. A spark plug comprising a shell having an insulator therein, a central electrode in said insulator, said insulator having a portion the sur- 2 assumes strength than the insulator surrounding the upper end of said electrode and serving to reduce the electrical stress at the expo ed surface thereor so as to prevent corona discharge or flashover.

4. A spark filua insulator assembly comprising an insulator having a central bore therein, and

' having a portion adapted to be exposed to the air in the use oi the plug, an electrode in said bore, and material of lower dielectric strength than the insulator surrounding the electrode throughout the said portion 0! the insulator so as to reduce the electrical stress in the exposed portion of the insulator and prevent corona discharge or nashover.

. TAINE G. MCDOUGAL'. 

